At the date of application the great majority of motor cars (“automobiles”, in US English) and other steerable, self-propelled land vehicles use an internal combustion engine (hereinafter an “ICE”) for their propulsion. Electric vehicles (sometimes referred to herein as “EVs”) are commercially available and are currently the subject of intensive research and development since it is widely supposed that they will within the foreseeable future supplant ICE-propelled vehicles, or at least take a greatly increased market share. The concept of a vehicle powered by an electric motor from on-board batteries is as old as the automobile itself. For most of the twentieth century, however, such vehicles struggled in most practical applications to compete with their ICE powered alternatives. But there are at the current point in time numerous incentives, both societal and technological, for greater adoption of EVs. It is hoped that EVs will, in comparison to ICE propelled vehicles, be less polluting, cheaper to run and quieter, among other advantages.
Arguably the biggest factors which have up to now favoured ICEs over electric propulsion systems for motor vehicles relate to battery technology. The term “battery” will be used herein in its conventional sense, to refer to a unit comprising one or more electrical cells to store energy and to supply it in electrical form. It should not be understood to encompass fuel cells, however. When used in EVs, batteries suffer from shortcomings at least in relation to:
(1) Energy density, typically expressed as the ratio of a battery's energy storage capacity to its mass. Volumetric energy density—energy storage capacity per unit volume—is also often referred to. On either measure, battery technologies have traditionally lagged behind ICEs and their fuel tanks. The mass and bulk of batteries that can be accommodated in a vehicle is limited. This—in conjunction with the limited energy density achieved by battery technology—limits the energy storage capacity that can be provided by a vehicle's battery, which in turn limits an EV's range—the distance it can be driven between battery charging sessions. To provide an EV with a range comparable to that of an ICE propelled vehicle typically requires a battery pack that is heavy and bulky—for some purposes prohibitively so. The mass of the battery pack may make up a substantial part of the mass of the vehicle, reducing the vehicle's acceleration and increasing its energy usage.
(2) Convenience of recharging. Refueling a conventional fossil-fuel driven motor car at a petrol station (gas station) is a quick and convenient, if expensive, process for the driver. Recharging an EV can be less convenient. The problem is partly one of time and partly of access to a suitable electrical supply. Due to the time taken to achieve a full charge, many EVs are at the time of writing intended to be charged overnight at the owner's home. Some commercially available vehicles claim to be capable of taking a partial charge in times under an hour. But the time and consequent inconvenience involved in recharging batteries remains an important impediment to uptake of the technology. The problem can be seen to be especially acute for those living in homes without off-street parking. Vehicle owners who have a driveway or garage to which they can lead a charging point connected to the electric mains may have little problem in charging their vehicle overnight. Consider however the problem faced by an urban driver who parks their car on the public street overnight. This individual will not normally be able to lead an electric cable from home to vehicle to carry out overnight charging, and nor will it be convenient for them to drive to a dedicated charging site and wait the considerable time needed for the battery to be recharged. Alternative solutions, such as the installation of chargers on residential streets, raise further problems of availability, cost and regulatory hurdles. In addition, some destinations, such as hotels, may have no charging options whatsoever.
(3) Power density. The rate at which energy can be delivered by a given battery is limited, which potentially limits the acceleration of an EV and its sustainable top speed. Using a battery to deliver power at the upper end of its operating envelope can lead to battery heating, which is inefficient and reduces battery lifetime. In designing batteries there is normally a compromise to be made between energy density and power density, but high values of both are desirable in most EVs.
(4) Cost. Battery technology is an area of intensive research and advances in this field in recent years have ameliorated problems of energy density and power density somewhat. It is now possible to achieve high performance and acceptable vehicle range in an electrically powered motor car, as exemplified by the well-known Tesla® range. These cars still have battery packs with a mass much greater than that of the engine and fuel tank of an equivalent petrol-driven car, but in addition the cost of the battery pack for such a vehicle is at the time of writing several times the cost of an ICE of similar performance, and the battery packs have a finite lifetime, requiring periodic replacement.